This invention relates generally to solar collectors and more particularly the type where each unit has a tubular housing configuration.
Efforts to achieve a useful conversion of solar energy into a more readily useable energy form have been intensified as the cost of other forms of useful energy has increased. The economics of the apparatus to be used, as well as any maintenance costs are major considerations in the choice of solar collector designs.
It is well known that a portion of the electromagnetic radiation received from the sun will pass through transparent materials such as glass, and will then heat an opaque object positioned behind the glass. Commonly the space between the transparent material and the object is evacuated to prevent heat loss by gaseous conduction and convection. U.S. Pat. No. 980,505 (Emmet) shows such arrangements as does U.S. Pat. No. 2,460,482 (Abbot). These patents disclose a tubular double-wall construction with a fluid within the inner tube. In the Abbot patent it is further proposed to coat the outer surface of the inner tube with a material highly absorbent of solar rays (see column 5, line 67 et seq).
When, as is frequently the case, it is desired to use a liquid as the medium to be heated, problems of liquid compatibility, as well as degradation of such organic components as gaskets and hoses used to contain the liquid have developed. Portions of the apparatus will also undergo cycles of expansion and contraction as the solar insolation changes which has resulted in breakage due to fatigue or poor design.
By combining a tube structure such as disclosed in FIG. 8 of the Emmet Patent with a coating on the outside of the inner tube as suggested by Abbot, a good beginning to a workable tubular solar collector is achieved.
More recently, U.S. Pat. No. 3,952,724 (Pei) while using an evacuated double-walled tubular housing such as above, proposed using a third tube to carry fluid into the inner end of the housing from which it can flow along the inside of the inner tube as disclosed by Abbot. This type of structure has the problem of loss of the total system fluid should any one of the tubular housings be broken. In addition, inlet and outlet manifolds are employed to which each collector must be joined in a fluid tight seal, with each seal a potential leakage point.